1. Field of the Invention
The invention relates to a method for monitoring the state of a fill level measuring device operating according to the radar principle, wherein the fill level measuring device comprises at least one transceiver unit for transmitting and receiving electromagnetic signals, wherein the fill level measuring device has at least one antenna for guiding, radiating and receiving electromagnetic signals, wherein the antenna has at least one interior space, and wherein the antenna is characterized by a transmission characteristic with regard to the transmission of electromagnetic signals. Furthermore, the invention relates to a fill level measuring device operating according to the radar principle, having at least one transceiver unit for transmitting and receiving electromagnetic signals and at least one antenna for guiding, radiating and receiving electromagnetic signals, wherein the antenna has at least one interior space, and the antenna is characterized by a transmission characteristic with regard to the transmission of electromagnetic signals.
2. Description of Related Art
In industrial measurement technology, radar fill level measuring devices are frequently used to determine the fill level of media, such as liquids, bulk solids or slurries inside containers such as tanks or silos. The transit time method used by the measuring device is based on the physical law that the transit distance, for example, of an electromagnetic signal is equal to the product of the transit time and the propagation speed. In the case of measuring the fill level of a medium—for example, a liquid or a bulk solid—in a container, the transit distance corresponds to twice the distance between an antenna radiating an electromagnetic signal and then receiving it again and the surface of the medium. The wanted echo signal—i.e., the signal reflected at the surface of the medium—and its transit time are determined by the so-called echo function or, respectively, the digitized envelope curve. The envelope curve represents the amplitudes of the echo signals as a function of the distance “antenna—surface of the medium.” The fill level can be calculated from the difference between the known distance of the antenna to the floor of the container and the distance between the surface of the medium and the antenna as determined by measurement. The transmitted and received electromagnetic signals are mostly microwave radiation.
Frequently, dielectric resonators are used as antennas. Such dielectric antennas have a resonance behavior similar to waveguides, however, since they do not have metal walls, they can radiate electromagnetic energy and thus act as antennas. A large number of construction forms are already known from different areas of technology, in each of which a dielectric material, such as a ceramic, is used for guiding and radiating the electromagnetic waves. Mostly, low-loss materials having low permittivity (another name for permittivity is dielectric conductivity) are used. TEFLON® (polytetrafluoroethylene) or polypropylene is used, for example. Another variation is that the microwave signals are guided along a rope or a rod.
Measuring devices are usually sealed against the process or the medium whose fill level is to be measured. Depending on the application, an additional encapsulation of the measuring device may be required opposed to a remote control body or opposed to an electronics unit removed from the process. A second sealing or encapsulation is particularly important; when, for example, the media is aggressive, harmful or explosive or when high pressures and high temperatures prevail in the process. Horn antennas for fill level measuring devices operating according to the radar principle, which are provided with protection and thus close the opening of the antenna, can be found, for example, in the U.S. Patent Application Publication 2003/0151560 A1 and corresponding U.S. Pat. No. 6,891,513 B2. A known method for testing if the seal facing the medium is sealed, is to provide an opening, such as a hole in the housing, between the two seals through which the medium or a gas mixture in the container escapes from the housing through a leakage and, thus, is an obvious signal. Such a procedure is questionable for dangerous and/or polluting substances or unacceptable in hygienic applications in the food and pharmaceutical industries. Other monitoring possibilities can be found in German Patent Application DE 103 52 471 A1 and corresponding U.S. Pat. No. 7,143,636 B2 or German Patent Application DE 10 2007 052 395 A1 and corresponding U.S. Pat. No. 7,827,866 B2.
For level measuring devices operating according to the radar principle, the patent U.S. Pat. No. 7,855,676 B2 describes a monitoring method in which a movable body is arranged in a hollow waveguide between a sealed horn antenna and an electronic unit. If the medium should enter into the waveguide through a leak, the body would then shift and thus change the transmission characteristic of the waveguide. However, this configuration is complex.